Optical disk recording method, optical disk reproducing method, optical recording medium using the method, and optical disk drive

ABSTRACT

An optical disk recording method of recording data on an optical disk by applying laser light to a track of the optical disk, includes the steps of a) driving a plurality of laser light sources in sequence in a time division manner, and applying a light flux to one track of the optical disk successively, and b) producing a plurality of rows of record marks in the one track.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disk recording method, anoptical disk reproducing method, an optical recording medium using themethod, and an optical disk drive, and, in particular, to an opticaldisk recording method, an optical disk reproducing method, an opticalrecording medium using the method, and an optical disk drive forrecording data at high density and reproducing the data thus recorded athigh density.

2. Description of the Related Art

Recently, as optical recording media (optical disk, magneto-optical diskand so forth) come to have large capacities, low costs, highreliabilities, and so forth, they have become used in various fieldssuch as recording/reproducing of image information,recording/reproducing of various code data in computer systems, and soforth.

Especially, optical disk drives are demanded to have further largecapacities, and need to use recording/reproducing methods of recordingdata at high density, and reproducing with high accuracy the datarecorded at high density.

As a recording/reproducing method for high-density data recording andhigh-accuracy data reproducing onto optical recording media, a method ofshortening of laser wavelength and relative shortening of spot diameterthrough improvement of numerical aperture (NA) have been performed, forexample.

Further, as a recording/reproducing method for high-density datarecording and high-accuracy data reproducing onto an optical recordingmedium of a magneto-optical recording system, shortening of record markthrough magnetic-field modulation recording has been performed, forexample.

In the related art, for optical recording media in the magneto-opticalrecording system, record marks through magnetic-field modulationrecording are recorded successively with portions thereof overlappedwith one another, a recording density in a track direction (referred toas a line density, hereinafter) is increased in comparison to a trackwidth, and, thus, the total recording density is increased. Further, thetrack width is shortened, a recording density in a radial direction(referred to as a track density, hereinafter) is increased, and, thus,the total recording density is also increased.

Thus, in order to increase a recording density, a line density isincreased, and/or a track density is increased.

However, when a track density is increased, in a case where interferenceof diffracted light is used as in a tracking error detecting systememploying a push-pull method, a tracking signal is degraded when thetrack width is shortened to be less than a predetermined width. Forexample, in a case where a laser wavelength is 650 nm and a numericalaperture is 0.6, the tracking signal is degraded when the track width isless than approximately 0.505 μm. Accordingly, there is a limit ofimprovement in track density.

When a line density is increased, because record marks are recordsuccessively with portions thereof overlapped with one another, crescentpatterns of the record marks come to be emphasized. Such crescent recordmarks have curvature at end portions thereof increased when the trackwidth increases, and portions which are not effective for reproducingincrease.

Especially, in a case of MSR (Magnetically-induced Super Resolution)medium of double mask type, the crescent pattern of a record mark and areproducing aperture (magnetic window) are exactly reverse in curvature.Accordingly, a resulting reproduced signal is degraded.

Thus, there is a limit of increase in recording density for each of themethods of increasing line density and increasing track density.

SUMMARY OF THE INVENTION

The present invention has been devised in consideration of those points,and, an object of the present invention is to provide an optical diskrecording method, an optical disk reproducing method, an opticalrecording medium using the method and an optical disk drive by which itis possible to record a plurality of rows of data in a single track, andreproduce the thus-recorded plurality of rows of data.

An optical disk recording method of recording-data on an optical disk byapplying laser light to a track of the optical disk, according to thepresent invention comprises the steps of:

a) driving a plurality of laser light sources in sequence in a timedivision manner, and applying a light flux to one track of the opticaldisk successively; and

b) producing a plurality of rows of record marks in the one track.

In this method, a plurality of laser light sources are driven insequence, and a light flux is applied to one track thereby in sequence.Then, a plurality of rows of record marks are produced in the track ofthe optical disk. At this time, because the plurality of laser lightsources are driven in sequence, there is no case where a plurality oflight fluxes are applied simultaneously. Accordingly, it is possible toproduce a plurality of rows of record marks in one track, avoidingoptical interference between the plurality of light fluxes.

In order to improve the recording density of the optical disk, it ispreferable that the step b) produces the plurality of rows of recordmarks in such a manner that the rows of record marks overlap partiallywith one another.

In this method, because the rows of record marks overlap partially withone another, it is possible to improve the line density. Further,because the plurality of rows of record marks are cut off by one anotherat the end portions thereof, it is possible to prevent the curvature ofthe record marks from increasing. Accordingly, it is possible to reduceproblems that the curvature of the record marks and the curvature of thereproduction aperture (window) do not coincide in MSR(Magnetically-induced Super Resolution) reproduction by which recordmarks smaller than the beam spot can be reproduced using a magneticmask. With regard to MSR reproduction of double mask type, see JapaneseLaid-Open Patent Application No. 7-244877.

In order to apply the method to a magneto-optical disk, a magnetic fieldmay be generated in the track of the magneto-optical disk by onemagnetic head.

In this method, it is possible to produce plurality of rows of recordmarks partially overlapping each other in one track of a magneto-opticaldisk through a simple configuration.

In order to reproduce data from an optical disk in which a plurality ofrows of record marks are recorded in one track, with high accuracy, anoptical disk reproducing method of reproducing data recorded on anoptical disk on which a plurality of rows of record marks are formed inone track, by applying laser light to the track, according to thepresent invention, comprises the steps of:

a) driving a plurality of laser light sources in sequence by a timedivision manner, and applying a light flux to one track of the opticaldisk successively;

b) causing reflected light to be generated from the plurality of rows ofrecord marks by the incident light flux successively, mixing thereflected light, and reproducing a signal changing according to statesof the plurality of rows of record marks; and

c) demodulating the data recorded in the track from the reproducedsignal according to a predetermined decoding algorithm.

In this method, a reading light flux is applied to one track, andreflected light is caused to be generated from a plurality of rows ofrecord marks to which the light flux is applied, in sequence. Thereflected light is signals changing according to states of the recordmarks to which the light flux is applied, and, from the signals, thedata recorded in the track of the optical disk can be obtained throughdemodulation according to a predetermined decoding algorithm.

Interference occurs between respective reflected light from record marksof a plurality of rows of record marks to which a light flux is applied.Accordingly, the signal changing according to the states of theplurality of rows of record marks contains interference between thereflected light from the plurality of rows of record marks, and the datamay be demodulated after sampling from the interference-containingsignal, through a Viterbi decoding algorithm.

In this method, because the data is demodulated, after sampling from thesignal containing interference between the reflected light from theplurality of rows of record marks, through a Viterbi decoding algorithm,it is possible to obtain more probable data through demodulation at highaccuracy.

Process can be performed easily when reflected light from record marksgenerated according to a light flux applied thereto successively in atime division manner by a plurality of laser light sources can betreated as g single reflected light. Accordingly, the signal changingaccording to the states of the plurality of rows of record marks isobtained from converting the reflected light generated in sequence fromthe plurality of rows of record marks by the light flux from the laserlight sources driven at a frequency equal to or higher than the Nyquistfrequency into electric signals, and mixing the electric signals; and

the data may be demodulated through a Viterbi decoding algorithm aftersampling at predetermined periods from the mixed signal.

In this method, because laser light sources are driven at a frequencyequal to or higher than the Nyquist frequency, a plurality of lightfluxes are applied to one track at very short intervals in sequence in atime division manner. Accordingly, a reproduced signal obtained fromconverting the reflected light and mixing it can be regarded as areproduced signal obtained from applying one light flux to the pluralityof rows of record marks simultaneously. Further, by demodulating thereproduced signal by the Viterbi decoding algorithm, it is possible toobtain more probable data at high accuracy through the demodulation.

Thus, it is possible to provide an optical disk recording method ofproducing a plurality of rows of record marks in one track of an opticalrecording medium in such a manner that the rows of record marks overlapwith each other partially, and to provide an optical disk reproducingmethod of reproducing data from the thus-recorded record marks.

In order to achieve an optical recording medium having an improvedrecording medium, in an optical recording medium according to thepresent invention on which data is recorded as a result of laser lightbeing incident in a recording track and a magnetic field being generatedtherein,

a plurality of rows of record marks are recorded in each track in such amanner that the plurality of rows of record marks partially overlap oneanother.

In the recording medium, because a plurality of rows of record marks arerecorded in each track, it is possible to improve a recording density ofrecord marks in a radial direction of the recording medium. Further,because the plurality of rows of record marks partially overlap oneanother, it is possible to further improve a recording density of recordmarks of the recording medium.

Further, because the plurality of record marks are cut off by each otherat the end portions thereof, it is possible to prevent the curvature ofthe record marks from increasing. Accordingly, it is possible to reducethe portions of the record marks which are not effective forreproduction.

The optical recording medium may be a magneto-optical disk.

This recording medium can be achieved from a magneto-optical disk, forexample, on which data can be rewriteable. The recording medium merelyshould be overwriteable, and may be achieved by a phase-change opticaldisk.

Thus, it is possible to provide an optical recording medium in which aplurality of rows of record marks are produced in one track of anoptical recording medium in such a manner that the rows of record marksoverlap with each other partially.

In order to achieve an optical disk drive which can record data in atrack of an optical disk at high density, an optical disk drive forrecording data on an optical disk by applying laser light to a track ofthe optical disk, according to the present invention, comprises alight-flux applying part driving a plurality of laser light sources insequence in a time division manner, and applying a light flux to onetrack of the optical disk successively; and

producing a plurality of rows of record marks in the one track.

In this optical disk drive, a plurality of laser light sources aredriven in sequence, and a light flux is applied to one track thereby insequence. Then, a plurality of rows of record marks are produced in thetrack of the optical disk. At this time, because the plurality of laserlight sources are driven in sequence, there is no case where a pluralityof light fluxes are applied simultaneously. Accordingly, it is possibleto produce a plurality of rows of record marks in one track, avoidingoptical interference between the plurality of light fluxes.

In order to improve recording density of the optical disk, it ispreferable that the light-flux applying part applies the light flux tothe track in a manner such as to produces the plurality of rows ofrecord marks in such a manner that the rows of record marks overlappartially with one another.

In this optical disk drive, because the rows of record marks overlappartially with one another, it is possible to improve the line density.Further, because the plurality of rows of record marks are cut off byone another at the ends thereof, it is possible to prevent the curvatureof the record marks from increasing. Accordingly, it is possible toreduce the portions of the record marks which are not effective inreproduction.

In order to apply this optical disk drive to a magneto-optical disk, theoptical disk drive may further comprise a magnetic-field generating partgenerating a magnetic field in the track of the magneto-optical disk byone magnetic head.

In this optical disk drive, it is possible to produce plurality of rowsof record marks partially overlapping each other in one track of amagneto-optical disk through a simple configuration.

In order to reproduce data from an optical disk in which a plurality ofrows of record marks are recorded in one track, with high accuracy, anoptical disk drive for reproducing data recorded on an optical disk onwhich a plurality of rows of record marks are formed in one track, byapplying laser light to the track, according to the present invention,comprises:

a reading light-flux applying part driving a plurality of laser lightsources in sequence by a time division manner, and applying a light fluxto one track of the optical disk successively;

a reproducing part causing reflected light to be generated from theplurality of rows of record marks by the incident light fluxsuccessively, mixing the reflected light, and reproducing a signalchanging according to states of the plurality of rows of record marks;and

a demodulating part demodulating the data recorded in the track from thereproduced signal according to a predetermined decoding algorithm.

In this optical disk drive, a reading light flux is applied to onetrack, and reflected light is caused to be generated from a plurality ofrows of record marks to which the light flux is applied, in sequence.The reflected light is signals changing according to states of therecord marks to which the light flux is applied, and, from the signals,the data recorded in the track of the optical disk can be obtainedthrough demodulation according to a predetermined decoding algorithm.

Interference occurs between respective reflected light from record marksof a plurality of rows of record marks to which a light flux is applied.Accordingly, the demodulating part may comprise:

a sampling part sampling at predetermined periods from the signalcontaining interference between the reflected light from the pluralityof rows of record marks; and

a Viterbi decoding part demodulating the data from the thus-sampled databy a Viterbi decoding algorithm.

In this method, because the data is demodulated by sampling from thesignal containing interference between the reflected light from theplurality of rows of record marks, through a Viterbi decoding algorithm,it is possible to obtain more probable data through demodulation at highaccuracy.

Process can be performed easily when reflected light from record marksgenerated according to a light flux applied thereto successively in atime division manner by a plurality of laser light sources can betreated as one reflected light. Accordingly, the reproducing part maycomprise a reproduced signal generating part converting the reflectedlight generated in sequence from the plurality of rows of record marksby the light flux from the laser light sources driven at a frequencyequal to or higher than the Nyquist frequency into electric signals, andmixing the electric signals.

In this optical disk drive, because laser light sources are driven at afrequency equal to or higher than the Nyquist frequency, a plurality oflight fluxes are applied to one track at very short intervals insequence in a time division manner. Accordingly, a reproduced signalobtained from converting the reflected light and mixing it can beregarded as a reproduced signal obtained from applying one light flux tothe plurality of rows of record marks simultaneously. Further, bydemodulating the reproduced signal by the Viterbi decoding algorithm, itis possible to obtain more probable data at high accuracy through thedemodulation.

Thus, it is possible to provide an optical disk drive for producing aplurality of rows of record marks in one track of an optical recordingmedium in such a manner that the rows of record marks overlap with eachother partially, and for reproducing data from the thus-recorded recordmarks.

In order to form a plurality of rows of record marks in one track of anoptical disk and record data at high accuracy, an optical disk recordingmethod of recording data on an optical disk by applying laser light to atrack of the optical disk, according to the present invention.,comprises the steps of:

a) applying a light flux to the track successively in such a manner thatthe thus-incident light flux crosses the track to-and-fro obliquely; and

b) producing a plurality of rows of record marks in the track.

In this method, a light flux is applied to one track successively.Thereby, a plurality of rows of record marks are formed in the track. Atthis time, because the plurality of laser light sources are driven insequence, there is no case where a plurality of light fluxes are appliedsimultaneously. Accordingly, it is possible to produce a plurality ofrows of record marks in one track, avoiding optical interference betweenthe plurality of light fluxes.

In order to reproduce data from an optical disk on which a plurality ofrows of record marks are formed in one track, at high accuracy, anoptical disk reproducing method of reproducing data recorded in a trackof an optical disk on which a plurality of rows of record marks areformed in one track, by applying laser light to the track, according tothe present invention, comprises the steps of:

a) applying a reading light flux to the track successively for theplurality of rows of record marks in such a manner that thethus-incident light flux crosses the track to-and-fro obliquely; and

b) causing reflected light to be generated from the plurality of rows ofrecord marks by the incident light flux successively, mixing thereflected light, and reproducing a signal changing according to statesof the plurality of rows of record marks; and

c) demodulating the data recorded in the track from the reproducedsignal according to a predetermined decoding algorithm.

In this method, a reading light flux is applied to one track, andreflected light is caused to be generated from a plurality of rows ofrecord marks to which the light flux is applied, in sequence. Thesereflected light is signals changing according to states of the recordmarks to which the light flux is applied, and, from the signals, thedata recorded in the track of the optical disk can be obtained throughdemodulation according to a predetermined decoding algorithm.

In order to form one row of record marks in one track of an optical diskand to enable downward comparability, the light-flux applying part maycause the positions of the plurality of rows of record marks to be equalto each other.

By causing the positions of the plurality of rows of record marks to beequal to each other by making the positions in the track to beirradiated by the plurality of laser light sources driven in sequence ina time division manner be equal, it is possible to treat the recordmarks produced in the track as one row of record marks. Accordingly, theoptical disk drive is downward compatible.

In order to reproduce data from an optical disk on which one row ofrecord marks are formed in one track and to enable downwardcomparability,

the reading light-flux applying part may drive one of the plurality oflaser light sources, and apply the reading light flux to the tracksuccessively; and

reproduce from a row of record marks recorded in the track by theincident light flux.

By reproducing from one row of record marks produced in one track bydriving one laser light source of the plurality of laser light sources,it is possible to reproduce from the one row of record marks formed inthe track. Accordingly, the optical disk derive is downward compatible.

An optical disk recording method of recording data on an optical disk byapplying laser light to a track of the optical disk, according toanother aspect of the present invention, comprises the steps of:

a) driving a laser light source and outputting the laser light;

b) controlling a position on which the thus-output laser light isincident in one track in a radial direction of the optical disk; and

c) producing a plurality of rows of record marks in the track, row byrow.

In this method, a laser light source is driven and the laser light isoutput therefrom. At this time, a position at which the thus-outputlaser light is applied to is controlled in one track, in a radialdirection of the optical disk. Because a plurality of rows of recordmarks are produced in the track, row by row, in sequence, it is possibleto produce a plurality of rows of record marks in one track with onelaser light source.

The present invention in the other aspect can also be applied to amagneto-optical disk.

Further, the plurality of rows of record marks may be produced such amanner that the positions of the plurality of rows of record marks aremade to be equal to each other in a rotation direction of the opticaldisk, and the record marks the positions of which are equal to eachother in the rotation direction are made to have the same state.Thereby, it is possible to achieve downward compatibility.

In order to reproduce data from an optical disk on which a plurality ofrows of record marks are formed in one track, at high accuracy, anoptical disk reproducing method of reproducing data recorded in a trackof an optical disk on which a plurality of rows of record marks areformed in one track, by applying laser light to the track, according tothe present invention, comprises the steps of:

a) driving a single laser light source and applying a reading light fluxto one track;

b) simultaneously causing reflected light to be generated from theplurality of rows of record marks by the applied light flux, mixing thereflected light, and reproducing a signal changing according to statesof the plurality of rows of record marks; and

c) demodulating the data recorded in the track from the thus-reproducedsignal according to a predetermined decoding algorithm.

In this method, a reading light flux is applied to one track, andsimultaneously causing reflected light to be generated from theplurality of rows of record marks by the applied light flux. Thereflected light is signals changing according to states of the recordmarks included in a portion of the track to which the light flux isapplied, and, it is possible to demodulate the data recorded in thetrack of the optical disk from these signals according to thepredetermined decoding algorithm.

In order to achieve an optical disk drive recording data in a track ofan optical disk at high density, an optical disk drive for recordingdata on an optical disk by applying laser light to a track of theoptical disk, according to the other aspect of the present invention,comprises:

a laser outputting part driving a single laser light source andoutputting the laser light; and

a light-flux controlling part controlling a position at which thethus-output laser light is applied to the optical disk in one track, ina radial direction of the optical disk, and

producing a plurality of rows of record marks in the track, row by row.

In this optical disk drive, a single laser light source is driven andthe laser light is output therefrom. At this time, a position at whichthe thus-output laser light is applied to is controlled in one track, ina radial direction of the optical disk. Because a plurality of rows ofrecord marks are produced in the track, row by row, in sequence, it ispossible to produce a plurality of rows of record marks in one trackwith a single laser light source.

In order to reproduce data from an optical disk on which a plurality ofrows of record marks are formed in one track, at high accuracy, anoptical disk drive for reproducing data recorded in a track of anoptical disk on which a plurality of rows of record marks are formed inone track, by applying laser light to the track, according to the otheraspect of the present invention, comprises:

a reading light-flux applying part driving a single laser light sourceand applying a reading light flux to one track;

a signal reproducing part simultaneously causing reflected light to begenerated from the plurality of rows of record marks by the appliedlight flux, mixing the reflected light, and reproducing a signalchanging according to states of the plurality of rows of record marks;and

a demodulating part demodulating the data recorded in the track from thethus-reproduced signal according to a predetermined decoding algorithm.

In this optical disk drive, a reading light flux is applied to onetrack, and simultaneously causing reflected light to be generated fromthe plurality of rows of record marks by the applied light flux. Thereflected light is signals changing according to states of the recordmarks included in a portion to which the light flux is applied, and, itis possible to demodulate the data recorded in the track of the opticaldisk from these signals according to the predetermined decodingalgorithm.

Thus, according to the other aspect of the present invention, it ispossible to provide an optical disk recording method of producing withone light source a plurality of rows of record marks in one track of anoptical recording medium in such a manner that the rows of record marksoverlap with each other partially, and to provide an optical diskreproducing method of reproducing with one light source data from thethus-recorded record marks.

Further, it is possible to provide an optical disk drive for producingwith one light source a plurality of rows of record marks in one trackof an optical recording medium, and for reproducing with one lightsource data from the thus-recorded record marks.

Other objects and further features of the present invention will becomemore apparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a block diagram roughly showing an example ofconfiguration of an opitcal disk drive in a first embodiment of thepresent invention;

FIG. 1B shows a block diagram showing an example of configuration of anoptical system shown in FIG. 1A;

FIGS. 2A, 2B and 2C show timing charts showing signal stringscontrolling data recording of the optical disk drive shown in FIG. 1A;

FIGS. 3A and 3B show examples of rows of record marks recorded in atrack of the optical disk shown in FIGS. 1A and 1B;

FIGS. 4A and 4B show examples illustrating relationships betweendisplacement of laser light sources and displacement of record marks;

FIGS. 5A through 5E show timing charts showing signal stringscontrolling data recording of rows of record marks shown in FIG. 3B;

FIGS. 6A, 6B and 6C show relationships between curvature of record marksand reproduction;

FIGS. 7A, 7B and 7C show examples of beam spots in a case where two beamspots are regarded as one beam spot;

FIG. 8 shows a block diagram showing an example of configuration of adata reproducing system;

FIGS. 9A through 9F show one example showing a relationship betweenrecord marks and clock signals;

FIG. 10A shows states and transitions thereof (clock pulse from even toodd) in interfered four marks;

FIG. 10B shows states and transitions thereof (clock pulse from odd toeven) in interfered four marks;

FIG. 11 shows states and transitions thereof (clock pulse from even toodd) in interfered six marks;

FIG. 12 shows states and transitions thereof (clock pulse from odd toeven) in interfered six marks;

FIG. 13 shows states and transitions thereof (clock pulse from even toodd) in interfered eight marks;

FIG. 14 shows states and transitions thereof (clock pulse from odd toeven) in interfered eight marks;

FIGS. 15A and 15B show examples showing states in interfered eightmarks;

FIG. 16 shows an example of relationship between states, statetransitions and record marks;

FIG. 17 shows a block diagram roughly showing an example ofconfiguration of an opitcal disk drive in a second embodiment of thepresent invention;

FIG. 18 shows a block diagram showing an example of configuration of anoptical system shown in FIG. 17;

FIGS. 19A and 19B show timing charts showing signal strings controllingdata recording of the optical disk drive shown in FIG. 17;

FIGS. 20A and 20B show an optical disk recording method of the opticaldisk drive shown in FIG. 17;

FIG. 21 shows an optical disk reproducing method of the optical diskdrive shown in FIG. 17;

FIGS. 22A, 22B and 22C show examples showing relationships betweenrecord marks and beam spots in the optical disk drive shown in FIG. 17;and

FIGS. 23A through 23F show one example indicating a relationship betweenrecord marks and clock signals in the optical disk drive shown in FIG.17.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A shows an optical disk drive in a first embodiment of the presentinvention.

The optical disk drive 1 includes a focusing servo system 6, a trackingservo system 7 and a feeding servo system 9 which control laser light toa desired position on an optical disk 32, a motor 2 which rotates theoptical disk 32, a rotating motor servo system 8 which controls arotation speed of the optical disk 32, an optical system 3 which appliesthe laser light to the optical disk 32 and outputs the reflected lightas a reproduced signal, a feeding motor 4 which moves the optical system3, a signal processing system 5 which receives the reproduced signal andperforms signal processing on the signal, and a drive controller 10which controls operations of the respective systems.

In FIG. 1A, the focusing servo system 6 controls the optical system 3 sothat the focus position of the laser light is always located on therecording surface of the optical disk 32. The tracking servo system 7controls the optical system 3 so that the laser light follows a track ofthe optical disk 32. The feeding servo system 9 controls the feedingmotor 4 so that the laser light moves to a desired track of the opticaldisk 32 at high speed.

Through the optical disk drive 1, it is possible to record a pluralityof rows of record marks on one track of the optical disk 32.

FIG. 1B shows an example of a configuration of the optical system 3 ofthe optical disk drive 1 shown in FIG. 1A.

This example has two optical systems.

In FIG. 1B, LD (Laser Diode) driving circuits 10 and 12 have pulse-likedriving signals A and B, such as those shown in FIGS. 2A and 2B,provided thereto at a time of data recording.

These driving signals A and B have phases different by 180°, and,accordingly, it can be said that pulse-like driving signals are providedto the LD driving circuits 10 and 12 alternately. Accordingly, the LDdriving circuits 10 and 12 cause LD 14 and LD 16 to which those circuitsare connected to emit light alternately.

Laser light emitted by the LD 14 is reflected by a beam splitter (BS)18, passes through a beam splitter 20, and is incident on an opticaldisk 32 through a lens 30.

Laser light emitted by the LD 16 passes through the beam splitter (BS)18, passes through the beam splitter 20, and is incident on the opticaldisk 32 through the lens 30.

The lens 30 condenses the provided laser light so as to cause the laserlight to focus on the surface of the optical disk 32, and forms a beamspot thereon.

When the optical disk 32 is a magneto-optical disk, the optical diskdrive includes a magnetic head having a magnetic-field signal shown inFIG. 2C provided thereto and generating a magnetic field on the surfaceof the optical disk 32.

FIGS. 3A and 3B show examples of record marks recorded in a track of theoptical disk shown in FIGS. 1A and 1B.

The recording method may be according to any one of optical modulation,magnetic-field modulation, and so forth.

FIG. 3A shows one track 40 of the optical disk 32. A row 42 of recordmarks are recorded in an upper half of the track in the figure while arow 44 of record marks are recorded in a lower half of the track in thefigure.

Each of these rows 42 and 44 of record marks consists of a plurality ofrecord marks 46 or 48, which are recorded rightward from the left endone 46 or 48 in sequence.

For example, when the record mark 46 a at the left end of the row 42 ofrecord marks is produced, the record mark 48 a at the left end of therow 44 of record marks is recorded with a portion thereof overlappingthe record mark 46 a.

Further, the record mark 46 b subsequent to the left end one is recordedwith portions thereof overlapping the record marks 46 a and 48 a.

Subsequently, the record marks 48 b, 46 c, 48 c, . . . are recorded insequence in the same manner.

When the optical disk 32 is the magneto-optical disk, the optical diskdrive applies light to a portion at which the record mark 46 or 48 isproduced, and, simultaneously, generates a magnetic-field region 50 suchas to at least cover thereby the portion irradiated by a beam spot.Thereby, at the portion irradiated by the beam spot, the record mark 46or 48 in which a direction of magnetic field is changed to the directionof the magnetic field of the magnetic-field region 50 is recorded. Themagnetic-field region 50 can be generated by one magnetic head.

A relationship between displacement of the recording marks 46 and 48 anddisplacement of the laser light sources will now be described briefly.

FIGS. 4A and 4B illustrate the relationship between displacement of therecording marks 46 and 48 and displacement of the laser light sources.

FIG. 4A shows a case where positions of laser light sources 200 aresame. In this confutation, by driving the laser light sources insequence, it is possible to produce record marks different in positionin one track 202, as shown in the figure.

FIG. 4B shows a case where positions of laser light sources 210 aredifferent. In this confutation, by driving the laser light sources insequence, it is possible to produce record marks same in position in onetrack 212, as shown in the figure.

When record marks same in position are produced as a result of the laserlight sources 210 being disposed differently in position as shown inFIG. 4B, and record marks same in position are assumed as the samerecords, it is possible to produce rows of record marks which can beregarded as one row of record marks in one track 212 using the pluralityof laser light sources 210.

Accordingly, the optical disk drive according to the present inventionis downward compatible. Further, at a time of reproduction, as a resultof one of the plurality of laser light sources being driven andreproduction being performed, the optical disk drive according to thepresent invention is downward compatible.

The number of rows 42, 44 of record marks recorded in one track is notlimited to two as in the example shown in FIG. 3A It is possible torecord rows of record marks corresponding to the number of LDs. Forexample, it is possible to record four rows of record marks as shown inFIG. 3B.

FIG. 3B shows one track 60 of the optical disk 32.

Rows 62, 64, 66 and 68 of record marks, in the order from the top in thefigure, are recorded in the track, as shown in the figure.

Each of these rows 62, 64, 66 and 68 of record marks consists of aplurality of record marks 72, 74, 76 or 78 which are recorded rightwardfrom the left end one 72, 74, 76 or 78 in sequence.

In a case where 4 rows of record marks are recorded in one track asshown in FIG. 3B, the optical disk drive is configured to have fouroptical systems, that is, four LD driving circuits and four LDs. FIGS.5A, 5B, 5C, 5D and 5E show an example of signals controlling datarecording in the optical disk drive in this case. FIGS. 5A, 5B, 5C, 5Dand 5E show signals controlling data recording for the rows of recordmarks shown in FIG. 3B.

For example, in the optical disk drive configured to include four LDdriving circuits and four LDs, pulse-like driving signals A through Dshown in FIGS. 5A through 5D are provided to the four driving circuits,respectively. As shown in FIGS. 5A through 5D, the driving signals Athrough D are provided to the four LD driving circuits so that the fourLDs emit light in sequence.

When the optical disk 32 is a magneto-optical disk, the optical diskdrive is configured to include a magnetic head having a magnetic-fieldsignal shown in FIG. 5E provided thereto and generating a magnetic fieldon the surface of the optical disk 32.

Thus, in a case where a plurality of rows of record marks are recordedin one track as mentioned above, it is possible to record a plurality ofrows of record marks in one track by providing LD driving circuits andLDs corresponding to the number of the rows of record marks.

Further, by producing record marks with portions thereof overlappingeach other by causing LDs to emit light in sequence, it is possible toavoid interference between laser light emitted by the LDs, and, also, toimprove the recording density. Further, because end portions of therecord marks are cut off by each other, it is possible to preventcurvatures of the record marks from being enlarged, and, to reduce theportions of the record marks not effective for reproduction.

With reference to FIGS. 6A, 6B and 6C, prevention of curvatures of therecord marks from being enlarged and reduction of the portions of therecord marks not effective for reproduction will now be described.

In FIG. 6A, when reproduction is performed from a record mark 80, theregion effective in the reproduction is a region enclosed by therectangular drawn by the thick line in the figure. The same manner isapplied to a case where a plurality of rows of record marks are recordedin one track as shown in FIG. 6B.

However, when a plurality of rows of record marks are recorded in onetrack with portions thereof overlapping one another laterally as shownin FIG. 6C, it is possible to enlarge the widths of the record marks tobe recorded first, in comparison to the case of FIG. 6B.

It is noted that, when the record marks recorded first have the largerwidths, no problem occurs because portions thereof are overwritten bythe record marks recorded subsequently.

Accordingly, the record marks 82 are prevented from having crescentshapes, and the regions effective in reproduction can be enlarged.

Description will now be made for reproduction of record marks recordedin one track as shown in FIGS. 3A and 3B with reference to FIG. 1B.

In FIG. 1B, at a time of data reproduction, the LD driving circuits 10and 12 have pulse-like driving signals A and B provided thereto such asthose to cause the LDs 14 and 16 to emit light in synchronization withthe record marks or pulse-like driving signals A and B provided theretosuch as those to cause the LDs 14 and 16 to emit light at a frequencyequal to or higher than the Nyquist band of the record marks.

For example, when pulse-like driving signals A and B such as those tocause the LDs 14 and 16 to emit light in synchronization with the recordmarks are provided to the LD driving circuits 10 and 12, the LD drivingcircuits 10 and 12 cause the LDs 14 and 16, to which they are connectedrespectively, to emit light alternately in synchronization with therecord marks.

When pulse-like driving signals A and B such as those to cause the LDs14 and 16 to emit light at a frequency equal to or higher than theNyquist band of the record marks are provided to the LD driving circuits10 and 12, the LD driving circuits 10 and 12 cause the LDs 14 and 16, towhich they are connected respectively, to emit light alternately at thefrequency equal to or higher than the Nyquist band of the record marks.

The laser light emitted by the LD 14 is reflected by the beam splitter18, passes through the beam splitter 20, and is applied to the opticaldisk 32 through the lens 30.

The laser light emitted by the LD 16 passes through the beam splitter18, passes through the beam splitter 20, and is applied to the opticaldisk 32 through the lens 30.

The lens 30 condenses the incident laser light so as to focus it on thesurface of the optical disk 32 and forms a beam spot thereon.

When the beam spot is formed on the optical disk 32, the light reflectedby the optical disk 32 passes through the lens 30, is reflected by thebeam splitter 20, and is incident on a beam splitter 22.

The light incident on the beam splitter 22 is split according to apolarization component, and is provided to a photodiode (PD) 24 or 26.The photodiode 24 or 26 converts the provided optical signal into anelectric signal, and outputs the electric signal to an amplifier 28.

The amplifier 28 amplifies the electric signal provided by thephotodiode 24 or 26 and outputs the amplified signal as a reproducedsignal.

In a case where reproduction is performed as a result of the LD 14 andLD 16 being caused to emit light alternately in synchronization with therecord marks, data can be demodulated as a result of sampling being madefrom the reproduced signal according to the driving signals A and Bprovided to the LD driving circuits 10 and 12.

However, in a case where reproduction is performed as a result of the LD14 and LD 16 being caused to emit light alternately at a frequency equalto or higher than the Nyquist band of the record marks, the reproducedsignal is an average of reproduced signals obtained by means of the LD14 and LD 16. Accordingly, two beam spots formed by the LD 14 and LD 16are regarded as one beam spot.

FIGS. 7A, 7B and 7C show examples of beam spots in a case where two beamspots are regarded as one beam spot.

In FIGS. 7A, 7B and 7C, beam spots 85, 86 and 87 have different sizesbecause the sizes of record marks are shown as being equal. However,actually, the beam spots 85, 86 and 87 are same in size.

FIG. 7A shows an example of a case (referred to as interfered fourmarks, hereinafter) in which four states of record marks are included inan area of the beam spot 85. Accordingly, the reproduced signal obtainedby means of the beam spot 85 is a signal mixed from reproduced signalsfrom the four states of record marks.

FIG. 7B shows an example of a case (referred to as interfered six marks,hereinafter) in which six states of record marks are included in an areaof the beam spot 86. Accordingly, the reproduced signal obtained bymeans of the beam spot 86 is a signal mixed from reproduced signals fromthe six states of record marks.

FIG. 7C shows an example of a case (referred to as interfered eightmarks, hereinafter) in which eight states of record marks are includedin an area of the beam spot 87. Accordingly, the reproduced signalobtained by means of the beam spot 87 is a signal mixed from reproducedsignals from the eight states of record marks.

Thus, the reproduced signal output from the amplifier 28 is a signalmixed from reproduced signals from a plurality of states of record marksincluded in the area of the beam spot. In other words, it is possible todemodulate the reproduced signal output from the amplifier 28 into theoriginal data, similarly to a case of a reproduced signal from anopitcal disk to which data is recorded after being modulated into aso-called partial response (PR) waveform.

For example, it is possible to detect the most probable data using aViterbi detector (most likely data detector) after sampling at apredetermined frequency from a reproduced signal from an optical disk towhich data is recorded after being modulated into a PR waveform.

FIG. 8 shows an example of a configuration of a data reproducing system.

In FIG. 8, an analog-to-digital converter 110 converts an analog signalof the reproduced signal provided by the amplifier 28 shown in FIG. 1Binto a digital signal. A filter may be provided between the amplifier 28shown in FIG. 1B and the analog-to-digital converter 110 for the purposeof shaping waveform as necessity arises.

The analog-to-digital converter 110 operates in synchronization with aclock signal from a clock generating circuit 130. That is, sampling ismade from the above-mentioned reproduced signal by the analog-to-digitalconverter 110, and the sampled values are output from theanalog-to-digital converter 110 in synchronization with the clocksignal.

For example, in a case where the LDs 14 and 16 are caused to emit lightalternately in synchronization with the record marks and datareproduction is performed, the clock generating circuit 130 outputs theclock signal according to the reproduced record marks.

In a case where the LDs 14 and 16 are caused to emit light alternatelyat a frequency equal to or higher than the Nyquist band of the recordmarks and data reproduction is performed, the clock generating circuit130 outputs the clock signal twice in frequency the clock signalcorresponding to the reproduced record marks.

The sampled values from the reproduced signal output from theanalog-to-digital converter 110 in sequence in synchronization with theclock signal are provided to the Viterbi detector 100 after undergoingwaveform equalization process and so forth by a digital equalizer (EQ)120.

The Viterbi detector 100 detects record data in accordance with aViterbi decoding (demodulating) algorithm from the sampled valuesprovided in sequence, and outputs it.

Although not shown in FIG. 8, the above-mentioned digital equalizer 120and clock generating circuit 130 operate also in synchronization withthe clock signal from the clock generating circuit 130.

The Viterbi detector 100 will now be described in detail.

The Viterbi detector 100 includes a branch-metric calculating unit (BM)101, an ACS (Add-Compare-Select) unit 102, a pass-metric memory (PMM)103, and a pass memory (PM) 104.

The branch-metric calculating unit 101 calculates branch-metric values(BM values) corresponding to a difference between each expected valuewhich should be obtained from the reproduced signal and the sampledvalue of the reproduced signal.

The ACS unit 102 adds each of the above-mentioned branch-metric valuesand the pass-metric value (PM value) preceding by one clock (calculatedlast time) and stored in the pass-metric memory (PMM) 103 together.Then, the ACS unit 102 compares each two of the thus-obtainedpass-metric values (PM values), selects the smaller one thereof. Thethus-selected pass-metric value (PM value) is stored in the pass-metricmemory 103 as a new pass-metric value. As a result of theabove-mentioned process being performed in sequence, the pass-metricvalue (PM value) becomes the accumulated value of the branch-metricvalues (BM values).

To thus select the smaller pass-metric value corresponds to select apath of state transition in the Viterbi decoding (demodulating)algorithm. Thus, the ACS unit 102 always selects the path such that thepass-metric value becomes minimum. Then, data (2-level data)corresponding to the selected path is provided to the pass memory (PM)104 from the ACS unit 104.

In the pass memory (PM) 104, the data corresponding to each selectedpath is shifted in sequence, and, also, the data corresponding to eachpath not selected in the process is dismissed in sequence. As a result,the data corresponding to the survivor path is output from the passmemory 104 as output data.

Thus, in a case where light is emitted from the LDs alternately in afrequency equal to or higher than the Nyquist band of the record marks,it is possible to accurately reproduce data-recorded at high density bydetecting most probable data using the Viterbi detector.

As a specific example, a procedure of reproducing the interfered eightmarks shown in FIG. 7C will now be described.

FIGS. 9A through 9F show one example of relationship between the recordmarks and clock signals.

FIG. 9A shows a clock signal corresponding to the row A of record marksrecorded on the optical disk. The period of the clock signal correspondsto the size of the record mark.

FIG. 9B shows a clock signal corresponding to the row B of record marksrecorded on the optical disk. The period of the clock signal correspondsto the size of the record mark.

As shown in FIGS. 9A and 9B, the rows A and B of record marks recordedin one track are different in phase by ½ period

When it is assumed that a clock signal twice in frequency the clocksignals shown in FIGS. 9A and 9B is a reference, the reproduced signalis obtained twice from one record mark.

For example, through the area 141 enclosed by the broken line shown inFIG. 9A, one state is obtained from one record mark when the clocksignal shown in FIG. 9A is used as a reference. However, through thearea 143 enclosed by the broken line shown in FIG. 9D, two states isobtained from one record mark when the clock signal shown in FIG. 9C isused as a reference.

Because FIGS. 9A through 9F shows the example of interfered eight marks,each beam spot includes four states of record marks.

This condition is shown in FIGS. 9D and 9E.

For example, the area 145 enclosed by the broken line shown in FIG. 9Acorresponds to the area of the beam spot, and includes four states ofrecord marks. The area 146 enclosed by the solid line shown in FIG. 9Dshows these four states of record marks. FIG. 9E also shows four statesof record marks similarly.

It is noted that each portion of the area of the beam spot correspondingto one clock pulse corresponds to the value “1” or “0”. Accordingly,because the area 146 corresponds to four clock pulses of the doubleclock signal shown in FIG. 9C and encloses the record mark of “1” forthe two clock pulses and the other record mark of “1” for the two clockpulses, the resulting value of the reproduced signal is “4” (sum of four“1”) as shown in FIG. 9D.

Further, each portion of the area of the beam spot corresponding to oneclock pulse corresponds to one state of a record mark. Accordingly, forexample, in FIG. 7C (interfered eight marks), the beam spot contains tworecord marks of the row A each for two clock pulses of the double clocksignal (2×2=4), one record mark for one clock pulse (1), another recordmark for two clock pulses (2) and another record mark for one clockpulse (1) of the row B, and, as a result, the total number of states ofrecord marks contained by the beam spot is eight (8).

However, in a case where reproduction is performed through emission ofthe LD 14 and LD 16 at a frequency equal to or higher than the Nyquistband of the record marks alternately, the reproduced signal output fromthe amplifier 28 is a mix of reproduced signals obtained by means of theindependent laser light A and B. Accordingly, the reproduced signaloutput from the amplifier 28 is the signal shown in FIG. 9F mixed fromthe signals shown in FIGS. 9D and 9E.

The reproduced signal output from the amplifier 28 has different statesof record marks for the interfered four marks, interfered six marks andinterfered eight marks. FIGS. 10A through 14 show the states of recordmarks different for the interfered four marks, interfered six marks andinterfered eight marks, and conditions of state transitions.

FIG. 10A shows the states and state transitions (even clock pulse→oddclock pulse) in the interfered four marks. FIG. 10B shows the states andstate transitions (odd clock pulse→even clock pulse) in the interferedfour marks. FIG. 11 shows the states and state transitions (even clockpulse→odd clock pulse) in the interfered six marks. FIG. 12 shows thestates and state transitions (odd clock pulse→even clock pulse) in theinterfered six marks. FIG. 13 shows the states and state transitions(even clock pulse→odd clock pulse) in the interfered eight marks. FIG.14 shows the states and state transitions (odd clock pulse→even clockpulse) in the interfered eight marks.

As shown in FIGS. 10A through 14, because the record marks arereproduced alternately according to the above-described double (twice)clock signal, the state transitions are different according to whetherthe odd clock pulse or even clock pulse is used for the reproduction atthe moments.

With reference to FIGS. 13 and 14, an example of demodulating thereproduced signal of the interfered eight marks will now be described.With regard to the interfered four marks and interfered six marks,demodulation can be performed by a similar manner.

As shown in FIGS. 13 and 14, there are sixteen states of record marks,and can be represented by a 4×4 matrix Sij. This matrix Sij representsthe state of the times t−3, t−2, t−1 and the state of the times t−2,t−1, t where the states of record marks in the interfered eight marksare defined as those of the times t−3, t−2, t−1, t, as shown in FIGS.15A and 15B.

The state transitions shown in FIGS. 13 and 14 are different accordingto whether the odd clock pulse or even clock pulse of theabove-described double (twice) clock signal is used at the moments. Forexample, when the clock pulse used shifts from the odd clock pulse tothe even clock pulse, the state transitions are transitions between rowsof the matrix as shown in FIG. 13. There are 32 of those transitions, asshown in FIG. 13, and can be represented by a 4×8 matrix Tij.

On the other hand, when the clock pulse used shifts from the even clockpulse to the odd clock pulse, the state transitions are transitionsbetween columns of the matrix as shown in FIG. 14. There are 32 of thosetransitions, as shown in FIG. 14, and can be represented by a 8×4 matrixTij.

At this time, the data of the time t−3 is fixed by the transition. Thethus-fixed data includes, in a case where the two rows A, B of recordmarks are recorded in one track, four types of data, i.e., (A, B)=(0,0), (1, 0), (0, 1), (1, 1). These four types of data are defined as data0, 1, 2, 3, in the stated order.

For example, in FIG. 13, in the leftmost column, the state transition(accented by a thick-line enclosure) from the top state to the secondfrom the top state is described. In this transition, the data of thetime t−3 is (0, 0) and the ideal sampled value is “1”, as shown in thefigure, the source (previous) state of (0, 0, 0, 0), as shown in thefigure, means that the states (a, b, c, d) of record marks shown in FIG.15A are (0, 0, 0, 0), and the destination (current) state of (0, 1, 0,0), shown in FIG. 13, means that the states(b, e, c, d) of record marksshown in FIG. 15A are (0, 1,0, 0), as shown in the figure.

In details, the above-mentioned condition where (a, b, c, d) are (0, 0,0, 0) of the previous state means the condition where the states of rowA of the times (t−3, t−2, t−1) are (a, b, b) and (0, 0, 0), and thestates of row B of the times (t−3, t−2, t−1) are (c, c, d) and (0, 0,0), as shown in FIG. 15A. Similarly, the condition where (b, e, c, d) ofthe current state are (0, 1, 0, 0) means the condition where the statesof row A of the times (t−2, t−1, t) are (b, b, e) and (0, 0, 1), and thestates of row B of the times (t−2, t−1, t−0) are (c, d, d) and therefore(0, 0, 0), as shown in FIG. 15A.

In this example, (a, b, c, d, e) are (0, 0, 0, 0, 1). Accordingly, theentire eight states of the current state are (a, b, b, e) in row A and(c, c, d, d) in row B, and, therefore, the ideal sampled value is(a+b+b+e)+(c+c+d+d)=(0+0+0+1)+(0+0+0+0)=1. Further, the data of the timet−3 is (a, c) and therefore (0, 0).

Therefore, in the transition in this example, the previous state is of(a, b, c, d)=(0, 0, 0, 0), and, therefore, the data of the time t−3 is(a, b)=(0, 0), and the current state is of (b, e, d, d)=(0, 1, 0, 0),and, therefore, the ideal sampled value is(a+b+b+e)+(c+c+d+d)=(0+0+0+1)+(0+0+0+0)=1.

FIG. 16 shows the above-described relationship of the states, statetransitions, and record marks.

Further, in FIGS. 13 and 14, the expected values through interference atthe time of the respective state transitions are indicated as idealsampled values, and are expressed by a matrix Phij. The matrix Phij is,same as Tij, a 4×8 matrix when the above-described clock pulse used ofthe double clock signal shifts from the odd one to the even one, but a8×4 matrix when the above-described clock pulse used of the double clocksignal shifts from the even one to the odd one.

Under the above-described conditions, data is demodulated from thesampled value yt obtained through interference by the Viterbi detector100 shown in FIG. 8. The sampled value yt is the sampled value of thereproduced signal provided to the BM 101.

The BM 101 calculates the branch metric value (BM value) correspondingto the difference between the sampled value yt and the ideal sampledvalue which is the expected value of each state transition, through thefollowing equation (1):BMij=(yt−Phij)² or |yt−Phij|  (1)

The branch metric value Bij can be expressed, same as the ideal sampledvalue, by a 4×8 matrix when the clock pulse used of the above-describeddouble clock signal shifts from the odd one to the even one, but by a8×4 matrix when the clock pulse used of the above-described double clocksignal shifts from the even one to the odd one.

The ACS 102 calculates the pass-metric value PMij by adding the BMij,and performs comparison and selection for the minimum pass-metric valuePMij.

It is noted that the pass-metric value PMij is the sum of the branchmetric values BMij in the paths through which transitions to the stateSij are made

According to Viterbi decoding, the path having the minimum pass-metricvalue PMij is determined as most probable.

Accordingly, PMij is defined using the following equations (2) and (3):

When clock pulse shifts from odd to even (only transition betweencolumns)PMij(t)=min[PMik(t−1)+BMim(t), PMil(t−1)+BMin(t)]  (2)where k=2mod(j,2), l=2mod(j,2)+1, m=2j, n=2j+1.

When clock pulse shifts from even to odd (only transition between rows)PMij(t)=min[PMkj(t−1)+BMmj(t), PMlj(t−1)+BMnj(t)]  (3)where k 2mod(i,2), l=2mod(i,2)+1, m=2i, n=2i+1.

Using the above equation (2) or (3), the pass-metric value PMij iscalculated, and is stored in the pass-metric memory 103. Then, thethus-stored pass-metric value PMij(t) is used, and the pass-metric valuePMij(t+1) is calculated using the equation (2) or (3).

Further, according to the path selected when the minimum comparison isperformed in the ACS unit 102, data is output to the pass memory 104. Inother words, one data 0, 1, 2 or 3 is selected for one state, and, thus,4×4 matrix data Dij is output to the pass memory 104.

Selection rules of output data according to selected path can beexpressed, according to FIGS. 13 and 14, by the following tables 1 and2: TABLE 1 At the time of shift of clock pulse from odd to even: FormerItem Latter Item i Selection Selection 0 0 2 1 0 2 2 1 3 3 1 3

TABLE 2 At the time of shift of clock pulse from even to odd: FormerItem Latter Item i Selection Selection 0 0 1 1 0 1 2 2 3 3 2 3

The pass memory 104 is designed to includes a plurality of stages ofshift registers, and, has a configuration of a matrix SRij in memorysystem expressed by a 4×4 matrix same as that of the state Sij. The passmemory 104 receives the data Dij according to the selected path from theACS unit 102, and shift registers in each stage of the pass memory 104have data provided thereto from the shift registers corresponding to thepreceding clock pulse. The data to be thus provided is defined,according to FIGS. 13 and 14, by the following equations (4) and (5):

At the time of shift of clock pulse from odd to even (only transitionsbetween columns):SRij(t)=SRik(t−1) when Dij=0 or 1;SRij(t)=SRil(t−1) when Dij=2 or 3  (4)where:

-   -   k=0, l=2, when j=0 or 1;    -   k=1, l=3, when j=2 or 3.

At the time of shift of clock pulse from even to odd (only transitionsbetween rows):SRij(t)=SRkj(t−1) when Dij=0 or 2;SRij(t)=SRlj(t−1) when Dij=1 or 3  (5)where:

-   -   k=0, l=2, when i=0 or 1;    -   k=1, l=3, when i=2 or 3.

According to the rules indicated by the equations (4) and (5), the shiftregisters have the data provided thereto from the shift registerscorresponding to the preceding clock pulse in sequence, and, all thedata coincide with each other through the sufficient number of stages ofshift registers.

That is, the data Dij of the shift registers of the matrix SRij,subsequent to a certain number of stages thereof, has the same data ofany of (0, 1, 2, 3) in all the elements thereof. Accordingly, it ispossible to demodulate the data of the rows A, B of record marksrecorded in one track under use of the double (twice) clock signal.

It is noted that, even in a case where the number of rows of recordmarks recorded in one track is increased, the data can be demodulatedthrough a similar process as a result of a matrix of m-dimensions beingused for representing the states, the number of which is increasedaccordingly.

It is noted that a basic principle of the above-described Viterbidecoding (demodulating) method is well-known (for example, see ‘DATAREPRODUCTION SYSTEM’, Continuation Application under 35 U.S.C. 111(a) ofInternational Patent Application No. PCT/JP99/01273, filed on Mar. 16,1999 by the present applicant).

In the above-described method in which a plurality of rows of recordmarks are recorded in one track and reproduction is made therefrom,because laser light has a superior coherent property, when a pluralityof laser light beams are caused to approach one another, beat occurs dueto interference therebetween. As a result, the intensity of the laserlight changes periodically.

Further, in a case where a single laser light beam is used forreproducing from a plurality of rows of record marks recorded, crosstalk occurs between a signal of a desired row of record marks and asignal of another row of record marks recorded in the same tracktogether. As a result, it becomes difficult to separate the signal ofthe other row of record marks recorded in the same track together fromthe signal of the desired row of record marks.

FIG. 17 shows an optical disk drive in a second embodiment of thepresent invention which has been devised in consideration of theabove-described points.

The optical disk drive 201 shown in FIG. 17 includes a focusing servosystem 206, a tracking servo system 207 and a feeding servo system 209which control laser light to a desired position on an optical disk 232,a motor 202 which rotates the optical disk 232, a rotating motor servosystem 208 which controls a rotation speed of the optical disk 232, anoptical system 203 which applies the laser light to the optical disk 232and outputs the reflected light as a reproduced signal, a feeding motor204 which moves the optical system 203, a signal processing system 205which receives the reproduce signal and performs signal processing onthe signal, and a drive controller 210 which controls operations of therespective systems.

In FIG. 17, the focusing servo system 206 controls the optical system203 so that the focus position of the laser light is always located onthe recording surface of the optical disk 232. The tracking servo system207 controls the optical system 203 so that the laser light follows atrack of the optical disk 232. The feeding servo system 209 controls thefeeding motor 204 so that the laser light moves to a desired track ofthe optical disk 232 at high speed.

Through the optical disk drive 201, it is possible to record a pluralityof rows of record marks in one track of the optical disk 232 bycontrolling the tracking servo system 207 for example.

FIG. 18 shows an example of configuration of the optical system 203shown in FIG. 17.

In FIG. 18, an LD (Laser Diode) driving circuit 212, at a time of datarecording, has a pulse-like driving signal shown in FIG. 19A providedthereto for example. The LD driving circuit 212 causes an LD 216,connected thereto, to emit light.

The laser light emitted by the LD 216 is incident on the optical disk232 after being transmitted by a beam splitter (BS) 220 and passingthrough a lens 230.

The lens 230 is controlled in a focusing direction and in a trackingdirection by a lens actuator not shown in the figure. The lens actuatoris controlled by the focusing servo system 206 and tracking servo system207, and controls the lens 230 in the focusing direction and in thetracking direction. As a result, the lens 230 condenses the incidentlaser light so as to cause it to focus on the surface of the opticaldisk 232, and forms a beam spot in a desired track of the optical disk232.

Through the optical disk drive 201, it is possible to record a pluralityof rows of record marks in one track of the optical disk 232 bycontrolling the lens actuator by the tracking servo system 207 andcontrolling the lens 230 in the tracking direction.

In a case where the optical disk 232 is a magneto-optical disk, theoptical disk drive 201 is configured to include a magnetic head whichhas a magnetic-field signal shown in FIG. 19B provided thereto, andgenerates a magnetic field on the surface of the optical disk 232.

An optical disk recording method using the above-described optical diskdrive 201 will now be described.

FIGS. 20A and 20B illustrate the optical disk recording method. In theexample shown in FIGS. 20A and 20B, two rows of record marks arerecorded in one track.

The recording method may be according to any one of optical modulation,magnetic-field modulation, and so forth.

First, as shown in FIG. 20A, the lens 230 is controlled in the trackingdirection so that the beam spot 235 follows an upper half, in thefigure, of one track 236 of the optical disk 232.

The beam spot 235 is incident in the upper half of the track 236successively as the optical disk 232 rotates, and records a row 237 ofrecord marks including record marks 237 a and 237 b.

For example, the row 237 of record marks are recorded in sequencerightward from the left-end record mark 237 a.

When the optical disk 232 has made one revolution and recording in theupper half of the track 236 has been finished through one circlethereof, recording of another row of record marks is performed in thelower half of the track 236 through one circle thereof in a subsequentrevolution or a later revolution.

As shown in FIG. 20B, the lens 230 is controlled in the trackingdirection so that the beam spot 235 follows the lower half, in thefigure, of the track 236 of the optical disk 232.

The beam spot 235 is incident in the lower half of the track 236successively as the optical disk 232 rotates, and records the row 238 ofrecord marks including record marks 238 a and 238 b.

For example, the row 238 of record marks are recorded in sequencerightward from the left-end record mark 238 a.

As shown in FIG. 20B, the row 237 of record marks and row 238 of recordmarks recorded in one track 236 are different in phase by ½ period.

Although FIGS. 20A and 20B illustrate the example where the two rows 237and 278 of record marks are recorded in one track 236, it is alsopossible to record three or more rows of record marks in one track in asimilar method.

Reproduction from the record marks recorded in one track as shown inFIG. 20B will now be described with reference to FIG. 18.

In FIG. 18, the LD driving circuit 212, when reproducing data, has apulse-like driving signal provided thereto for causing the LD 216 toemit light in synchronization with the record marks.

For example, when having the pulse-like driving signal provided theretofor causing the LD 216 to emit light in synchronization with the recordmarks, the LD driving circuit 212 causes the LD 216, connected thereto,to emit light in synchronization with the record marks.

The laser light emitted by the LD 216 is transmitted by the beamsplitter 220, and is incident on the optical disk 232 through the lens230. The lens 230 condenses the provided, laser light so as to cause itto focus on the surface of the optical disk 232, and forms a beam spotin a desired track of the optical disk 232.

In the optical disk drive 201, the lens actuator is controlled by thetracking servo system 207, for example, and, thus, the lens 230 iscontrolled in the tracking direction so that the beam spot follows thecenter of one track of the optical disk 232

FIG. 21 illustrates an optical disk reproducing method using the opticaldisk drive 201.

The example shown in FIG. 21 is one example in which reproduction isperformed from two rows of record marks recorded in one track of theoptical disk 232.

As shown in FIG. 21, the lens 230 is controlled in the trackingdirection so that a beam spot 235 follows the center of one track 236 ofthe optical disk 232.

The beam spot 235 is incident at the center of the track 236successively as the optical disk 232 rotates, and causes reflected lightto be generated from the rows 237, 238 of record marks simultaneously.That is, a reproduced signal obtained through the beam spot 235 is oneobtained from mixing reproduced signals from the rows 237, 238 of recordmarks.

In FIG. 18, when the reflected light from the optical disk 232 passesthrough the lens 230, it is reflected by the beam splitter 220, and isincident on a beam splitter 222. The reflected light incident on thebeam splitter 222 is split according to a polarization componentthereby, and is provided to a photodiode (PD) 224 or 226.

The photodiode 224 or 226 converts the provided light signal into anelectric signal, and outputs the electric signal to an amplifier 228.The amplifier 228 amplifies the electric signal provided by thephotodiode 224 or 226, and outputs a reproduced signal.

FIGS. 22A, 22B and 22C show examples illustrating relationships betweenbeam spots and record marks.

In FIGS. 22A, 22B and 22C, beam spots 285, 286 and 287 have differentsizes because the sizes of record marks are shown as being equal.However, actually, the beam spots 285, 286 and 287 are same in size.

FIG. 22A shows an example of a case (referred to as interfered fourmarks, hereinafter) in which four states of record marks are included inan area of the beam spot 285. Accordingly, the reproduced signalobtained by means of the beam spot 85 is a signal mixed from reproducedsignals from the four states of record marks.

FIG. 22B shows an example of a case (referred to as interfered sixmarks, hereinafter) in which six states of record marks are included inan area of the beam spot 286. Accordingly, the reproduced signalobtained by means of the beam spot 86 is a signal mixed from reproducedsignals from the six states of record marks.

FIG. 22C shows an example of a case (referred to as interfered eightmarks, hereinafter) in which eight states of record marks are includedin an area of the beam spot 87. Accordingly, the reproduced signalobtained by means of the beam spot 287 is a signal mixed from reproducedsignals from the eight states of record marks.

Thus, the reproduced signal output from the amplifier 228 is a signalmixed from reproduced signals from a plurality of states of record marksincluded in the area of the beam spot. In other words, it is possible todemodulate the reproduced signal output from the amplifier 228 into theoriginal data, similarly to a case of a reproduced signal from anopitcal disk to which data is recorded after being modulated into apartial response (PR) waveform.

For example, it is possible to detect the most probable data using aViterbi detector after sampling at a predetermined frequency from areproduced signal from an optical disk to which data is recorded afterbeing modulated into a PR waveform.

An example of a configuration of a data reproducing system fordemodulating such a reproduced signal is shown in FIG. 8.

The configuration and operations of the system shown in FIG. 8 have beendescribed above, and duplicated description will be omitted.

As a specific example, a procedure of reproducing the interfered fourmarks shown in FIG. 22A will now be described.

FIGS. 23A through 23F show one example of relationship between therecord marks and clock signals.

FIG. 23A shows a clock signal corresponding to the row A of record marksrecorded on the optical disk. The period of the clock signal correspondsto the size of the record mark. FIG. 23B shows the row A of record marksrecorded on the optical disk.

FIG. 23C shows a clock signal corresponding to the row B of record marksrecorded on the optical disk. The period of the clock signal correspondsto the size of the record mark. FIG. 23D shows the row B of record marksrecorded on the optical disk.

As shown in FIGS. 23A and 23C, the rows A and B of record marks recordedin one track are different in phase by ½ period

When it is assumed that a clock signal (double clock signal) twice infrequency the clock signals shown in FIGS. 23A and 23C is a reference,the reproduced signal is obtained twice from one record mark.

For example, through the area 141 enclosed by the broken line shown inFIG. 23B, one state is obtained from one record mark when the clocksignal shown in FIG. 23A is used as a reference. However, through thesame area 141, two states are obtained from one record mark when theclock signal shown in FIG. 23E is used as a reference.

Because FIGS. 23A through 23E show the example of interfered four marks,each beam spot includes four states of record marks.

This condition is shown in FIG. 23F.

For example, the area 143 a enclosed by the broken line shown in FIG.23B and the area 143B enclosed by the broken line shown in FIG. 23Dcorrespond to the area of the beam spot, and includes four states ofrecord marks. Accordingly, the reproduced signal output from theamplifier 228 is shown in FIG. 23F which is a mix (sum) of the statesshown in FIGS. 23B and 23D.

It is noted that the area 143 a includes one record mark of upper halfas shown in FIG. 22A having the state of “1” for two clock pulses of thedouble clock signal and corresponds to the reproduced signal value of“2”, while the area 143 b includes respective portions of two recordmarks of lower half as shown in FIG. 22A having the states of “1” and“0” each for one clock pulse of the double clock signal, respectively,and corresponds to the reproduced signal value of “1”. The reason whythe area 143a corresponds to the value “2” is that the area of therecord mark enclosed by the area 143 a is twice the area of each ofrespective portions of the record marks enclosed by the area 143 b, asshown in FIG. 22A.

The reproduced signal output from the amplifier 228 has different statesof record marks for the interfered four marks, interfered six marks andinterfered eight marks. The states of record marks different for theinterfered four marks, interfered six marks and interfered eight marks,and conditions of state transitions are shown in FIGS. 10A through 14,description thereof having been made, and duplicated description thereofbeing omitted.

Further, an example of reproduction of interfered eight marks has beendescribed with reference to FIGS. 13 and 14, and duplicated descriptionthereof will be omitted.

As described above, with regard to the interfered four marks andinterfered six marks, demodulation can be performed by a similar manner.

Further, it is noted that, even in a case where the number of rows ofrecord marks recorded in one track is increased, the data can bedemodulated through a similar process as a result of a matrix ofm-dimensions being used for representing the states, the number of whichis increased accordingly.

The present invention is not limited to the above-described embodiments,and variations and modifications may be made without departing from thescope of the present invention.

The present application is based on Japanese priority application No.11-345499, filed on Dec. 3, 1999, the entire contents of which arehereby incorporated by reference.

1-6. (Cancelled)
 7. An optical recording medium on which data isrecorded as a result of laser light being incident in a recording trackand a magnetic field being generated therein, wherein a plurality ofrows of record marks are recorded in each track in such a manner thatsaid plurality of rows of record marks partially overlap one another. 8.The optical recording medium as claimed in claim 7 comprising amagneto-optical disk. 9-20. (Cancelled)
 21. An optical recording mediumhaving laser light applied to and a magnetic field generated to arecording track so as to have data recorded thereto, wherein a pluralityof rows of record marks are formed in each track. 22-27. (Cancelled)